Start by trying to turn the wheel with the bobbin and the auto-thread cassette removed. Look inside to see if there is any thread caught anywhere both in the bobbin area and also in the complicated mechanism where the auto-thread cassette fits.
- If it is still stiff, remove the needle plate, which also holds-down the cover over the bobbin carrier. (You may need to use a coin and a pair of pliers to undo the 2 screws as there is very limited vertical clearance at this point)
- Remove the bobbin carrier, noting which way it fits. Thread and fluff can build up in this area.
- Thread might have become wound round under the rotating component into which the bobbin carrier fits.
- Put one drop of oil in the edge of the large screw in the centre of the rotating component and turn the wheel a few times. Put another drop of oil in the same place and continue to turn the wheel.
- Look carefully with a small flashlight into the case and see if there is any fluff or thread bits caught in the feed dogs mechanism.
- Put one drop of oil onto any joint between moving parts that you can see.
Now, if there is no improvement and you cannot see any threads caught, you need to see if there is anything wrapped round the bevel gear drive under the bobbin carrier rotating mechanism.
If you go to "How to fix Brother's CS-8060 bobbin auto winder to work?" thread on this forum there is a description of how to separate the 2 halves of the case. (I apologize for omitting to say about Large +Screw #6, located in the top of the case where the Auto-Thread cassette fits)
- Splitting the case enables you to have a good look around under the mechanisms and check for any thread or fluff. Put one drop of oil on any moving joint.
- Rotate the wheel, being careful to keep your fingers out of the way as you do so. Take careful note of the area below the bobbin mechanism, where there is a bevel gear that turns the drive through 90 degrees.
- There is also an eccentric part that moves the feed dogs mechanism. Check there is nothing clogging-up this bit. You can use Lithium-based grease very sparingly to lube the cams and plastic/nylon components
- Put one drop of oil onto the needle-shaft, above where it slides through the mounting. Turn the wheel a few times and put just one more drop of oil.
- Look at the crank mechanism that drives the needle-shaft and apply one drop of oil on any moving joint or component.
- While you are in this area check that all the bits that move do so freely, especially the little joints of the auto-thread.
- Have a good look around the area of the bobbin winder, up by the wheel. - Has the bobbin winder drive rubber gone gooey? Is there anything caught up there?
Hopefully you may find what is causing the stiffness. It is not really likely that one of the shaft bearings is tight but you can put a little oil each side of the bearing and continue to turn the wheel to see if this frees-up any.
Take care to wipe any excess oil off, particularly where it might get on anything you sew.
Don't get oil on the drive belt and don't blow any fluff up into the motor area or, especially, the speed-sensor disk (which is a black thing with fine slots.
Assemble the components carefully. You might take a few cell-phone pictures to help remember how things go.